In electric discharge machining by the wire electric discharge machine and the engraving electric discharge machine, a voltage is applied between an electrode and a workpiece in an insulative machining fluid to cause a discharge, so that the workpiece is machined by the thermal action (vaporization and melting) and the dynamical action (discharge impact pressure). The machining fluid passes an electric discharge machining region to become contaminated machining fluid containing machining dust caused by the electric discharge machining, and is collected in a contaminated fluid tank. The contaminated machining fluid taken from the contaminated fluid tank is filtered by a filter to remove the machining dust and then stored in a clean machining fluid tank. Then, the clean machining fluid is supplied from the clean machining fluid tank to the machining region again.
FIG. 3 is a schematic diagram showing a generally used conventional system for processing machining fluid by circulatoin. A filter 4 is disposed in a clean fluid tank 2 and is hence submerged in the clean machining fluid. The machining fluid having passed an electric discharge machining region is collected in a contaminated fluid tank 1 and then introduced from the contaminated fluid tank 1 to a filter attachment pipe 5, which is arranged at a center of the filter 4 and serves as a machining fluid passage, by a pump 3. The machining fluid is then introduced from the attachment pipe 5 into the clean fluid tank 2 through the filter 4 to thus remove machining dust from the machining fluid. The machining fluid is drawn up from the clean fluid tank 2 by the pump 2 and the clean machining fluid with the machining dust removed is supplied again to the machining region.
After a long-term use, the filter 4 is clogged with the machining dust to become unfit for use. Therefore, it is necessary to exchange the filter 4 for a new one. For exchanging the filter 4, it is necessary to remove a nut 6 threadedly fitted on a screw at an end of the filter attachment pipe 5 and a seal washer 7 and to pick up the filter 4 vertically along the filter attachment pipe 5, so that the filter 4 is removed from the filter attachment pipe 5. Since the filter 4 is clogged with the metal machining dust and submerged in the machining fluid, the filter 4 contains the machining fluid and therefore is very heavy (e.g., about 20 kg), thus requiring heavy labor in the exchanging operation of the filter.
Further, since both the contaminated fluid tank and the clean fluid tank are usually disposed under the electric discharge machine and other mechanical parts are disposed over the tanks, a spacial restriction makes it difficult to exchange the filter.